The continuing acceleration in computing power demand and supply, with ever-more transistors operating at ever-higher speeds, has brought about the need for liquid cooling of the computer chips. A chip today might contain hundreds of millions of transistors and emit two hundred watts of heat energy (twice the heat output of a 100-watt incandescent light bulb, emitted from a much smaller surface area), and the chip needs intense cooling. When chips are ganged and collected in complex devices, the amount of heat given off can be quite large, so that rooms full of electronic equipment become uncomfortably warm and are difficult to air-condition. Sometimes special air-conditioning must be arranged. For example, large areas of the floor have been converted to air-conditioning vents in some electronic installations
Until recently, most electronic equipment, including most computer equipment, has been air-cooled. Air is free and air leaks are unimportant. However, air is a gas, therefore of low density, and therefore its cooling ability is limited. Liquids, being much denser than gasses, are more efficient at removing heat. Anyone who has sat in a cold bath knows that water is very chilling, even when it is at the same temperature as air that is comfortably cool, and water is said to be 3600 times more chilling than air. Besides possessing a greater heat capacity per unit of volume, liquids can boil to remove heat even faster than it can be removed by conduction to a liquid.
Electronic systems have been cooled by chilled water, but the fluid of choice may be a fluorine-based liquid, similar to the refrigerant FREON. Such liquid has the advantage of being an electrical insulator (water is not an insulator, and will cause short circuiting if it reaches the circuits). Fluorine-based liquids are available from the 3M company and come in different varieties having various physical properties. These fluorine-based liquids have low boiling points and, if they leak out of a system, will evaporate instantly instead of dripping onto any electronic components, which happens with water. These liquids also have low freezing points, and systems equipped with them need not be protected from freezing, for example during transport. Water, unlike most liquids, expands on freezing rather than contracting like most liquids, which is the cause of winter-time burst pipes.
Another advantage of fluorine-based liquids for cooling is proportionality. These liquids have physical properties that permit a system using them to be scaled down to a smaller size. When a water-cooled system is scaled down in size, the surface tension of the water can interfere with the functioning of the system. Viscosity is another physical property of a liquid that can become more or less influential when a device is scaled to a different size.
The conventional electronic cooling system includes a rack containing the computer and/or other electronic equipment to be cooled, a heat exchanger for removing heat from the liquid after it is heated by the electronics, a pump, and a storage tank. The heat exchanger, pump, tank, and connections together are referred to as a cooling distribution unit or CDU.
A tank is needed in a CDU system because the coolant liquid can leak out of the system, either through defects or because of coupling and uncoupling components. Because of this, some backup volume of the liquid must be provided, and the tank holds this extra volume of liquid. As liquid leaks out, it is replaced by air and/or gaseous vapor.
Another reason that a tank is needed is because of thermal expansion and contraction. Even if no fluid ever leaks out and no air ever gets in, the volume of the liquid will change, and one way to accommodate this change is to have a tank that is not quite full of liquid, but instead has some gas in it. The gas, being compressible, will allow for liquid volume change.
Because it is preferable to have a small amount of gas in a tank, and it is likely that a more-than-preferable amount of air or other gas will get into the system, the tank should be able to supply liquid even when there is a large amount of gas in the tank. The supply of liquid from the tank should be steady, without any interruptions and without air bubbles leaving the tank.
Because of the large amounts of heat to be removed from modern electronic equipment, the flow rate in a cooling system might be as much as ten gallons per minute, or even more. At such high flow rates, bubbles of any air or vapor that become entrained in the stream of coolant liquid are liable to be carried through the tank to the exit port and then to the pump. Bubbles in the pump can cause it to stop operating by vapor lock. Even a small bubble can also cause a temperature spike in the equipment to be cooled, because a small hot area in contact with a bubble cannot eliminate heat as well as adjacent areas that are immersed in cooling liquid.
Bubbles can enter with the liquid at the tank's entry port, or they can be generated inside the tank by turbulence caused by the high velocity of the liquid inside the tank and pump. Bubble formation is aggravated by low levels of the liquid in the tank, which increases the chances of air being entrained in surging or whirling liquid. Vortices (whirlpools or miniature tornadoes) tend to form at the exit port when the exit velocity is high. A vortex has a core of gas, and can suck the gas directly into the exit flow from the tank.
Cavitation is also a concern. Cavitation is the formation of bubbles of vacuum or the attenuated vapor of the liquid itself (as opposed to air or other gas which has entered the system). Cavitation, like the formation of vortices, is associated with high liquid flow rates. (When rowing a boat, cavitation can be seen next to the oar blades when the oars are forced through the water at high speed, as voids next to the oar; but cavitation does not appear at low speed.) Cavitation is harmful because the bubbles of cavitation collapse quickly soon after they are formed, and the sudden collapse creates a shock wave in the liquid that can damage pumps or other equipment.
Thus, there has been a need for a tank that does not generate or pass bubbles into the tank outlet pipe, even when the level of liquid in the tank drops.
In addition, there has been a need for a cooling distribution unit that can operate in various positions, as the electronic equipment to which it is attached is moved to various positions (for example, a computer might be laid over onto its side while running). In some circumstances, electronic equipment and its attached cooling system might even rotate continuously or rock intermittently, as on a boat that rolls but does not pitch significantly.
A bubble which is in a liquid reservoir can pass to the circulatory system if a change in orientation causes the bubble to extend to the reservoir outlet, where the bubble can get into the system. There has been a need for an electronic-cooling-system liquid reservoir that can be rotated during operation without introducing bubbles into the fluid circulation system or changing its operation, and this need has not been met by the conventional rectangular tank with inlet and outlet arranged in random positions.
The traditional tank, with a simple inlet orifice and a simple outlet orifice, does not meet these needs. If the outlet is uncovered, gas will be sucked directly into the system; vortices are certain to form at high flow rates; and bubbles may be sucked into the exit orifice even if it is located at the bottom of the tank.